Asphalt is a bitumen which is available in many varieties depending upon its natural origins and on the industrial process used in its production. Chemically, bitumens, such as asphalt, are a mixture of aliphatic, aromatic and naphthenic hydrocarbons with high molecular weight and small quantities of organic acids, bases and heterocyclic components containing nitrogen and sulfur. Asphalt is a colloidal substance, in which, the dispersed phase consisting of asphaltene, is covered by a protective phase of polar resins in complexes called micelles which are dispersed in a phase consisting of oils. The chemical nature of the various phases is not readily definable. Generally, however, the nucleus has characteristics that are more aromatic than naphthenic, the protective resins are prevalently naphthenic and the oils, which cover the micelles have a paraffinic character. The properties of bitumens, such as asphalt, are strictly associated the balance of the percentages of its components. Due to the difficulty of performing an exact chemical analysis, a classification is normally accepted which is based upon fractionated precipitation of the bitumen using selective solvents and an elution of the soluble in a chromatographic column (ASTM S2007-75 Method). Identification of an asphalt or bitumen is made by combining the results of this analysis with the values of penetration, softening and penetration index. Physically bitumen is a visco-elastic material, with viscous flow under slow stress and at high temperatures and more elastic behavior under rapid stress at low temperature.
Due to its wide availability, relatively low price and ease of application, asphalt has found a widespread use as a road-building material, notwithstanding its visco-elastic behavior. Intrinsic limitations accompany the use of asphalt as a road-building material. Asphalt demonstrates softening and unwanted flow at high temperatures, brittleness and unwanted fracturing at low temperatures, poor mechanical and elastic characteristics and a tendency to aging with exposure. Mineral aggregate is frequently added to asphalt to modify its rheology and temperature susceptibility. Roads frequently are laid with a base course and binder layers that insulate the upper asphalt surface from the ground, and the upper asphalt road surface develops extremely hot temperatures during the summer months and extremely cold temperatures during the winter months. The rheology of asphalt is such that, notwithstanding mineral additives, at high temperatures, it will flow in response to stresses imposed by vehicular traffic and develop "ruts" that not only provide unacceptable surface for vehicular travel, but provide localized areas of unacceptable thickness which crack under loads imposed by vehicular traffic at cold temperatures of winter and form pits (referred to as "chuckholes").
The efforts made to solve these shortcomings include the addition of modifying polymers to the asphalt used in the construction of roads. The selection of asphalt modifying polymers, however, must satisfy a number of requirements. The modifying polymer must not cause unwanted increases in the viscosity of the asphalt in its molten state or interfere with the use of existing road-building processes and apparatus. The modifying polymer should be sufficiently compatible with the asphalt as to not cause phase separation. In addition, the modifying polymer must be cost effective, that is, the polymer should improve the rheology and strength of the asphalt with which it is mixed sufficiently that any increased road costs imposed by use of the modifying polymer are recovered through reduced road maintenance and resurfacing costs.
A number of polymers have been considered for use in modifying bitumens and asphalts for use in roads.
Atactic polypropylene (APP) contributes a continuous matrix when it is mixed with bitumen at levels of 20 to 30 parts per hundred parts of bitumen. At such levels APP contributes no significant influence on the viscosity of the resulting mixture in its molten state. The viscosity remains acceptably low and dispersion of the APP in the bitumen is readily achieved. However, the resulting mixture is susceptible to phase separation, particularly during hot storage and requires special precautions and use. While bitumen modified with APP demonstrates increased stiffness, high ring and ball and low penetration values, the cohesion forces and mechanical characteristics of the APP-modified material remain poor and no appreciable improvements in the mechanical properties are achieved. Through the intrinsic stiffness of the resulting APP-modified bitumen, stresses of a given magnitude must be exceeded to obtain deformation, but because of its plastic behavior the deformation is generally irreversible. When subjected to temperature cycling, APP-modified bitumen behaves in an unsatisfactory manner and cracks even with small flexures. Although APP is a saturated polymer, it also demonstrates de-polymerization phenomena with a consequent loss of continuous lattice and separation of phases.
Thus, as an asphalt modifier, APP can contribute benefits in the manufacturing of modified asphalt in its easy solubility and low viscosity and in the application of the modified asphalt through the ease with which it is rolled and applied; however, the modification of asphalt with APP is not generally satisfactory because of the lack of an appreciable improvement in the elasticity and the mechanical properties of the resulting modified asphalt.
Asphalts have been more successfully modified for road applications through the use of thermoplastic (non-vulcanite) elastomeric block copolymers. Thermoplastic elastomeric block copolymers differ in molecular structure from typical plastic and commercial rubbers, which are generally homopolymers or random copolymers, in that they generally comprise a thermoplastic end block polymer, that is, chemically bound to and interconnected by an elastomeric mid block polymer. It has been known for many years that bitumens such as asphalt, may be modified with thermoplastic elastomeric block copolymers to increase their softening temperature, reduce their cold flow, improve their low temperature flexibility, improve their elastic recovery and improve their resistance to deformation. Such block copolymers can be blended with asphalt and using conventional road-building machinery, are capable of satisfactory hot storage in the modified state, and provide a sufficient improvement and the rheology and physical characteristics of the modified asphalt in a sufficiently low quantity and at a sufficiently low cost as to provide a cost effective improvement in the resulting asphalt road.
As noted above, thermoplastic elastomeric block copolymers generally comprise two incompatible polymers, a thermoplastic end block polymer, typically polystyrene, chemically joined with one of several elastomeric mid block polymers. Asphalt modifying block copolymers are sold commercially by the Shell Chemical Company of Houston, Tex., under its registered trademark, KRATON, and by EniChem Elastomeri Srl, of Milano, Italy, under its registered trademark Europrene.
Such block copolymers combine high tensile strength and flow resistance as a result of the polystyrene end blocks and high elasticity, cold temperature flexibility and fatigue resistance as a result of their elastomeric mid blocks. In use, the block copolymers tend to provide an elastic lattice interconnected by domains formed by their thermoplastic end blocks. Since the lattice structure is the result of physical rather than chemical forces, it may be destroyed either by dissolving the copolymer in a solvent or by heating it beyond the glass transition temperature of its thermoplastic end blocks. Upon evaporation of the solvent or cooling below the glass transition temperature of its thermoplastic end blocks, a structure may be re-imparted to the block copolymer. Such block copolymers are thus recyclable.
Thermoplastic block copolymers which can be successfully used with bitumens include styrene-butadiene-styrene copolymers (SBS), styrene-isoprene-styrene copolymers (SIS) and styrene-ethylenebutylene-styrene copolymers (SEBS). In addition to the traditional ABA-type tri-block polymers, such copolymers are available in the radial (A-B).sub.n and a di-block (A-B) structures. Prior to processing, the polystyrene end blocks of such copolymers are associated in rigid domains through physical cross-linking to yield a continuous three dimensional network. During processing in the presence of heat and shear or solvent, the polystyrene domains soften and permit flow and after cooling, reform to lock the interconnecting elastomeric network in place. As noted above, styrene domains impart high tensile strength to the resulting structure and the elastomeric mid block polymers impart elasticity, cold flow flexibility and fatigue resistance. Of these block copolymers, the SBS elastomers have been most frequently used to modify and improve asphalts for road construction.
When such SBS copolymers added to asphalt, a phase version takes place, and the asphalt is absorbed by a comparatively minor portion of SBS, which swells substantially. In preparation of modified asphalt, the molten asphalt softens the styrene end blocks of the SBS copolymer to allow a homogeneous polymer blend to be formed. High temperatures such as from 284.degree. F. (140.degree. C.) to 356.degree. F. (181.degree. C.), which are well above the styrene glass transition temperature of about 212.degree. F., permit fast polymer dissolution; however, mixing temperatures are kept below 390.degree. F. (199.degree. C.) to avoid unacceptable polymer degradation, and storage temperatures are generally maintained below 320.degree. F. (160.degree. C.).
High shear mixers such as the Siefer Trigonal and System Villas mixers are preferably used in blending the SBS copolymer and asphalt, in conjunction with internally agitated tanks. Such systems can produce several thousand gallons of modified asphalt per hour.
The modification of asphalt with SBS copolymer provides a significant improvement in the theology of the modified asphalt by providing reduced temperature susceptibility and increased flexibility at low temperatures and better resistance to flow and deformation at high temperature. Modification of the asphalt binders with SBS copolymer further improves tensile strength and the stiffness modulus at high temperatures, adhesion between the asphalt and the aggregate, and greater resistance to surface abrasion. At 75.degree. C. (167.degree. F.), unmodified AC5 asphalt has a viscosity of about 100 poise. The addition of about three percent (3%) SBS modifier (by weight) increases the viscosity of the modified asphalt about ten-fold, the addition of six percent (6%) SBS modifier (by weight) increases the viscosity of the modified asphalt well over 100-fold, and the addition of 15 percent (15%) SBS modifier (by weight) increases the viscosity of the modified asphalt well over 1000-fold.
Whenever composition ingredients are expressed in percentages, it is to be understood that the expressed percentage is the percent by weight of the resulting composition, unless otherwise stated. Where compositions are expressed in parts, it is to be understood that they are expressed in parts per hundred part rubber by weight.
Up to temperatures of 70.degree. C. (158.degree. F.), the physical and rheological properties of modified asphalt are highly dependent on the level of the SBS modifier used. Tests of the penetration index of asphalt modified with an SBS modifier indicate little improvement in penetration index below three percent, but a rapid improvement in penetration index between three and eight percent levels. Modified asphalts with over about eight percent of SBS modifier, however, demonstrate only modest improvements in penetration index, indicating that the addition of SBS modifier to asphalt in levels of six to eight percent are efficient for improvement of the temperature susceptibility of the modified asphalts. Tests of the stiffness modulus of modified asphalts indicate that improvements in the stiffness modulus are not significant until the percentage of SBS modifier nears six percent and that the stiffness modulus continues to improve as higher and higher levels of SBS modifier are used. As little as three percent SBS modifier in asphalt can improve the tensile strength, low temperature flexibility and rutting resistance of asphalts; however, it is generally considered that the additional improvements in temperature susceptibility associated with six percent or greater SBS asphalt loading are required.
In hot climates, aggregate, such as crushed limestone particles having diameters in the range of one-half to three-quarters of an inch in diameter, commonly called "chips", are added to the upper surface of an asphalt road to reduce its surface temperature and temperature susceptibility by isolating the asphalt surface from the effects of solar radiation, and to improve traction. Retention of such road chips on the road surface is substantially improved with modification of the surface asphalt by SBS modifiers. The modified asphalt demonstrates improved adhesion with the chip particles and provides a stronger, more ductile and elastic interface between the chips and the road surface. When SBS modified asphalt is used, road tests demonstrate that road surfaces formed with SBS modified asphalt and chips have a substantially longer life compared with unmodified asphalts where such surfaces fail to retain the chips.
More recently, it has become desirable to use ground rubber instead of aggregate as an extender for asphalt in road construction. Used tires provide a plentiful source of rubber and the use of old tires for ground rubber in road surfaces solves an environmental disposal problem. The use of ground rubber as an extender for asphalts, however, reduces the physical strength of the asphalt because, it is believed, of the reduced adhesion between the asphalt, reground rubber and any aggregate used in the road surface. Thus, the use of reground rubber in asphalt accentuates the susceptibility of the asphalt to rutting through its decreased flow resistance.
While SBS block copolymers are generally preferred for the modification of asphalts, styrene-isoprene block copolymers and linear styrene-ethylene/butylene styrene block copolymers all find use as asphalt modifiers.
U.S. Pat. Nos. 4,596,839 and 4,962,136 disclose the improvement of the elastomer compositions by an additive including particulate polytetrafluoroethylene and an amount of particulate molybdenum disulfide effective to provide uniform mixing of the polytetrafluoroethylene and the elastomer composition. These patents further disclose a new composition can comprise about 25 to about 80 percent polytetrafluoroethylene and about 1 to about 30 percent molybdenum disulfide by weight, with the balance of elastomer. The patents also disclose preferable compositions including about 2 to about 6 percent of polytetrafluoroethylene that is fibrillatable and fibrillated in the composition when combined with an effective amount of molybdenum disulfide. The polytetrafluoroethylene-molybdenum disulfide additives are disclosed as being useful in polymers, generally known as rubbers, including natural rubber and synthetic rubber elastomers and other polymers capable of forming elastic solids with similar properties. More specifically, such elastomers include, in addition to natural rubber, styrene butadiene rubber, acrylonitrile butadiene rubber, butyl rubber, ethylene-propylene rubber, polyurethane elastomers, CIS-polybutadiene polychloriprene, poly(epichlorohydrin), polyacrylate, silicone rubbers, poly(fluorinated hydrocarbons), olefin polysulfide, polyisoprene and the like. It is also disclosed that such compositions can also include plasticizers and softeners, extenders, reclaimed rubber, fillers, reinforcing fillers, coloring agents, antioxidants, accelerators and vulcanizing actuators.